3aPA10. Investigation of a narrow-band signal structure in the vortex ring
noise.

Experimental investigations of the vortex ring noise carried out recently
in an anechoic chamber showed the presence of a narrow peak in averaged spectra
near the frequency f=1200 Hz the width of which was 200 Hz. Separate
realizations recorded previously by tape recorder were used for averaging. The
main questions are: What is the mechanism of narrow peak appearance? What is
the reason for such a peak width? In the first part of this work a careful
comparison between the experimental data and the simplest theoretical model is
conducted. Vortex ring modes that could be responsible for the radiation are
obtained. However, the investigation of separate time realization considered in
the second part of this work showed that the process of sound radiation by
vortex ring is of a more complex nature than it was initially expected. The
registered radiation appeared to have the form of short time flashing
oscillations (wave trains) not connected in phase with each other. Observed
peculiarities of the acoustic signal are inherent to the narrow-band random
process, but not the mixture of the harmonic component and broadband noise as
it seemed earlier, i.e., the mechanism of vortex ring sound generation is
probably connected with an interaction between vortex core modes and the
turbulence near the core.